The present invention concerns a method for detecting defective fuel rods in a formation of complete fuel assemblies of water-cooled nuclear reactors.
Nuclear reactor fuel rods are the smallest unit in the construction of a reactor core; they contain the nuclear fuel, usually in the form of pellets, and give off the nuclear fission heat generated in them to the surrounding cooling water. It is here of the greatest importance that as far as possible, no radioactive fuel substances pass into the cooling water. This is achieved principally by a design of the fuel rods such that a cladding tube or casing with end caps welded-on gas-tight, surrounds the nuclear fuel and shields the latter as well as the fission products generated in it from direct contact with the cooling water.
During extended operation of the reactor, it can, however, not be completely precluded that isolated fuel rods develop leaks, so that cooling water can enter and possibly also radioactive material can escape. The cooling water purification systems provided in every nuclear plant, are laid out so that they can remove a certain amount of radioactive material from the cooling water circulation. If however, the radioactivity becomes so high that these systems are overloaded, there is no other recourse than to examine the core for defective fuel assemblies or fuel rods during shutdown periods.
This examination can first be performed purely visually by inspecting the fuel assemblies removed from the reactor core, usually under water by means of an underwater television camera. By this method, mainly highly defective fuel rods can be found, particularly such which are located in the outer rows of the fuel assemblies. However, it is very difficult to visually inspect fuel rods by this approach, which are located in the interior of a fuel assembly. To this must be added that it is also important to identify such nuclear fuel rods as defective which have only minor leaks, which cannot be detected externally by purely visual inspection.
In this connection, it has already been proposed to provide fuel rods in the interior with auxiliary devices which change upon the entry of water in such a manner that the change can be ascertained from the outside, e.g., magnetically. Such devices, however, increase the manufacturing costs of the fuel rods and can, of course, have sources of trouble of their own.
The problem therefore arose to find a method by which it is possible to ascertain the entry of water in defective fuel rods which have no special internals for this purpose. In this connection, it is particularly important that such an examination can be performed on the completed fuel assembly. The technical means required for disassembling and re-assembling irradiated fuel assemblies would be very great. In completely disassembling and re-assembling a fuel assembly, the great danger exists furthermore that individual fuel rods and the support structure (spacers) might get damaged. It should also be possible to carry out such examinations within the time available for fuel changing. For economic reasons this time must be kept as short as possible.